He may have done some nipping, or he may just have snagged his fins on his decor. He looks like both my HMs. Their fins regrow to a certain point then they destroy them again. I am not certain if it is the colouration of his fins, or if there is any actual rot.
AQ salt is the only thing I could suggest to treat him with. It looks like you've got a good schedule for WC's. Aside from perhaps bumping up the amount you change every week, I'm not sure what else to say. Just remember if you do go the aq salt route you want to make sure not to use it for more than 10 days. And might want to float him in a smaller container, or a spare QT tank for daily water changes. I have no idea if AQ salt will affect the ottos.
As for the lethargy and not eating, I'm not sure. The only thing I could suggest is soaking his food in some garlic juice and see if that will stimulate his appetite. When you thaw a few worms or brine shrimp just add a bit to the water.
Conservative dosing for AQ salt is 1 tsp/gal. Pre-mix it. You can go up to 3 tsp/gal, but it's recommended you do so slowly, upgrading the concentration every 12 hours.
Stress coat may also help. And our resident guru swears by mixing up IAL (indian almond leaf) or crushed dried oak leaves. Tannin water is supposed to help them. I know my girls seem to enjoy the darker water.